In the manufacture of high density integrated circuits, and particularly when MOS devices therein are scaled down to deep submicron design rules, the problem of hot electron reliability becomes a big concern for device engineers. Hot electron reliability refers to a condition where electrons/hole pairs are generated in a silicon substrate of an MOS transistor as a result of the high electrical fields produced between the gate electrode and the substrate at the edge of the source and drain regions. After generation, these hot electrons move swiftly toward the oxide-silicon interface and gain enough energy to surmount the silicon-silicon dioxide barrier and get trapped into the oxide. This entrapment in turn causes an increase of threshold voltage over time, thereby resulting in a significant long term reliability problem.
One of the techniques used to reduce the maximum vertical electrical field at the edge of gate electrode is to create a so-called gate bird beak region by the re-oxidation of the polysilicon gate and the adjacent source and drain regions of the MOS transistor. This process reduces the maximum vertical electrical field under the gate edge, and in addition, it reduces the gate-to-drain overlap capacitance. However, the gate bird beak encroachment can degrade the transconductance of submicron MOS devices and impact upon their subthreshold swing/threshold voltage.